You can mirror the exhaust object, but it won't get you anywhere, since the game only recognizes carname_KIT00_EXHAUST_A. It would be a good idea if the game would recognize carname_KIT00_LEFT_EXHAUST_A and carname_KIT00_RIGHT_EXHAUST_A, but those do not exist.

I can rotate the mount points to make the exhausts exit on the side, but the exhaust flames still exit horizontally. So that's why I attached them to the body on my Viper and SLR.